Teletext is an ancillary service of television channels designed to send written information in addition to video information. This written information is encoded in the form of digital data packets, commonly called teletext packets, that are broadcast at regular time intervals corresponding to the frame return between two images. One to eighteen teletext packets are thus broadcast about every 20 ms.
A television screen displays a teletext page of 25 lines, each with 40 characters. However, a teletext page is defined in practice by more than 32 teletext packets numbered X0 to X31, the contents of which are defined by the ETS standard. For each page, the following packets are transmitted. A packet X0 called a header packet that contains information especially on the number of the page to which it pertains is transmitted. At most 25 packets X1 to X25 called normal packets contain the characters to be displayed on the television screen with each packet corresponding to a line to be displayed are transmitted. In addition, packets known as non-displayable packets, X26, X27, X28, X29, X30 or X31, contain inter alias information for the shaping of the characters to be displayed and information on links towards other teletext pages are transmitted.
A full teletext program of a television channel comprises, for example, 500 pages that are associated, as the case may be, to form logic sets known as magazines. Each magazine comprises a variable number of teletext pages. For example, the television station may propose four magazines whose themes are, respectively, sports, finance, international news and weather. The total time to broadcast a teletext program is about 40 seconds and all the teletext pages are broadcast in cycles. In other words, one page is broadcast approximately every 40 seconds. This enables the television station to very regularly update the information content of its pages or to create animation effects.
To display a desired page, a teletext decoder of a television receiver first seeks the desired page by decoding the page number associated with each packet X0 received. Then the decoder copies all the packets of the desired page in a display memory. This display memory is read permanently by display means of the screen of the television set to display its contents on the screen.
Most usually, when the teletext service of a television channel is transmitted, the decoder automatically searches for page 100 and, as soon as it is received, displays it automatically. This page is, for example, an introduction page of the program. The page 100 shows the different magazines as well as the numbers of the first page of each magazine. To access a particular magazine, the user then keys the page number associated with the first page of the magazine desired into a remote control device or a keypad.
The user can also access a special page of a magazine by keying in the corresponding number of the desired page. This is of course possible only if the user knows the number of the desired page. The contents of a page having a given number is likely to change between two broadcasts of the same program. Furthermore, one page number may correspond to different magazines from one television channel to another. It is therefore not always easy for the user to retrieve the number of a page dealing with a desired subject.
There are existing navigation systems used to access a page dealing with a desired subject, without it being necessary to have any knowledge of the corresponding page number. A first navigation system is the FLOF system commonly used in France, England and Spain. The FLOF system is based on a color choice system. At the bottom of each page, the 24th line displayed on the screen has several labels, each corresponding to a link towards another page of the program. The 24th line displayed corresponds to the broadcast packet X24.
A label is a string of characters comprising eventually one or more words by which the user can identify the contents of the corresponding page. A label may also contain the number of the corresponding page, encoded in decimal form so that it can be understood by the user. The FLOF system associates a color code with each label. This color code corresponds, for example, to the background color on which the label is displayed.
To request the display of one of the pages whose label is displayed at the bottom of the page being displayed, the user simply chooses the color code associated with the chosen label. This choice is made, for example, by the remote control or the control keypad associated with the television set which, in this case, has colored buttons.
Then, in the control packet X27 of the page being displayed, the decoder will read the page number associated with the chosen label. The packet X27 indeed contains the page numbers associated with each label of the corresponding packet X24 of the page being displayed. These page numbers are encoded in the form of binary numbers. The desired page is finally loaded, namely searched for and copied into the display memory by the decoder and then displayed on the screen.
The FLOF navigation system is practical because it requires no knowledge of page numbers to display and consult a desired page. However, the number of links or labels displayed on one page is limited to six and, most usually, only four are used. It is therefore sometimes necessary to display several unwanted pages before displaying a desired page.
For example, if a user wishes to consult a page about the Paris stock exchange, he may have to first display the first page of the finance magazine, and then the first page of a section on the different international stock exchanges before he can finally display the desired page that concerns the Paris stock exchange. In certain cases, it may be necessary to display up to ten pages before obtaining the desired page. This means that a relatively lengthy time is needed to access certain pages.
A navigation system such as the FLOF system is also a closed system because the contents of the packets X24 and X27 associated with each teletext page of the program correspond to choices made by the supplier of the service. The user cannot choose his own links according to his own requirements.
A second navigation system is the table of pages (TOP) system, which is commonly used in German-speaking countries. With each set of pages or each program, a supplement is transmitted that includes non-displayable packets pertaining to the hierarchical ordering of the pages. These packets contain a definition of the links associated with each page. These packets are stored at each reception of a program.
With a system of this kind, this hierarchical ordering is done at three levels. Each magazine is divided into a set of a variable number of sections, with each section being divided into a variable number of pages. When a desired page is being displayed, the decoder displays, for example, at the bottom of the page on the 24th line, the labels “previous page”, “next page”, “next section”, and “next magazine”. When the user has made his choice of a colored button associated with the different labels or else by the controlled buttons for moving and selecting a label, the decoder associates the choice made with a corresponding page number contained in the hierarchical ordering packets.
Finally, the decoder searches for the desired page and stores it, as soon as it is received, in the display memory. With a navigation system like the TOP system, the user can thus go more easily from one page to the next one or from one section to the next one. However, if the user is interested only in a first section and a third section, he still has to request the display of the second section before accessing the third one.
Thus, present-day navigation systems for teletext services are easy to use because it is not necessary to know the number of a desired page to gain access to it. However, their use most commonly requires fairly lengthy periods of time to access the desired information. It is often necessary to load and display several pages before obtaining a desired page.
To limit the time taken to load a teletext page, it is possible to use a decoder with a buffer memory whose size is sufficient to store several teletext pages, even all the pages of a program broadcast by a television channel. This is especially the case with high-end television sets. In this case, when the teletext service is transmitted, the decoder first stores all or part of the program and then displays the introduction page. When a desired page has to be displayed, it is simply copied from the buffer memory into the display memory. This is a fast process.
In parallel, the contents of the buffer memory are refreshed continuously as the teletext pages are received. The display of a desired page is immediate because it is no longer necessary to wait for the reception of this desired page to be able to display it. However, if the television set is used with a TOP navigation system or a FLOF system, the user will nevertheless have to possibly display several pages on the screen before reaching the desired page. This is annoying to the user and unnecessary.